Frequency to digital converters and time to digital converters are useful in a number of signal processing applications. For example, frequency to digital converters are commonly used in frequency locked loops and can be used in phase locked loops. FIG. 1 is a block diagram illustrating a typical frequency locked loop. The output of a digital frequency synthesizer 100 and the output of a frequency to digital converter 108 are differenced and passed to a digital filter 102. The output of the digital filter goes to a digital to analog converter (DAC) 104. The output of the DAC is passed to a voltage controlled oscillator (VCO) 106, which outputs an analog output signal. The feedback loop is completed when the analog output of the VCO is passed to the frequency to digital converter.
FIG. 2 is a diagram illustrating the functional behavior of a typical frequency to digital converter. The frequency to digital converter determines cycles per period and can be employed in a number of systems. Sometimes the number of cycles that occur in a period is fractional. The ability to determine the fractional number determines the resolution of the frequency to digital converter. Improving the resolution of the frequency to digital converter improves the performance of various systems that incorporate the frequency to digital converter, such as the frequency locked loop shown above. Attempts to improve the resolution include using a chain of inverters to generate additional oscillations for each cycle and counting the additional oscillations to determine the fractional number. However, this approach may be affected by process variation in semiconductor fabrication, and resolution may be limited by propagation time in the inverter chain. It would be useful if other techniques could be developed to more accurately perform frequency to digital conversion.